なぜなら
なぜなら · nazenara · JLPT N3 · conjunction
because
なぜ (why) + なら (if): if you ask why, here is the reason
なぜ means why and なら means if it is, so なぜなら opens the answer to a why question. It usually begins a sentence and ends with から or ので: なぜなら雨だから, because it is raining.